This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography of reticle patterns in a manner that reduces the so-called Coulomb effect.
Substantial development effort is being expended currently to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography technology capable of transferring fine patterns having minimum feature sizes of less than 0.1 xcexcm (i.e., below the 0.1 xcexcm rule). Among several microlithography techniques that have been considered, charged-particle-beam (CPB) microlithography (especially electron-beam microlithography) having a throughput sufficiently high for use in the mass production of semiconductor chips (especially memory chips) is an attractive candidate. This technique, termed xe2x80x9cdivided-reticle-reductionxe2x80x9d electron-beam (EB) microlithography, uses an xe2x80x9cEB stepper.xe2x80x9d In preliminary tests of a prototype EB stepper, divided-reticle-reduction EB microlithography has exhibited a capability of transferring patterns having critical dimensions of 0.1 xcexcm or smaller.
Conventional EB microlithography techniques include the so-called xe2x80x9ccell-projectionxe2x80x9d technique that differs from so-called xe2x80x9cdirect-writexe2x80x9d techniques. In the cell-projection technique a desired pattern is formed on the substrate (xe2x80x9cwaferxe2x80x9d) by projecting and connecting together basic pattern-element shapes termed xe2x80x9ccells.xe2x80x9d A variety of such basic shapes are defined on an aperture plate upstream of the substrate, and the cells are selected and projected in a xe2x80x9cmix-and-matchxe2x80x9d manner to reconstruct the pattern cell-by-cell on the substrate. Each cell typically is very small, e.g., about 5-xcexcm square, requiring a very small-diameter beam.
In contrast to the xe2x80x9ccell-projectionxe2x80x9d apparatus, an EB stepper configured to perform divided-reticle-reduction microlithography utilizes a reticle that defines pattern elements configured as the elements are to be projected onto the substrate. (Hence, xe2x80x9cmix-and-matchxe2x80x9d reconstruction of pattern elements on the substrate, which is slow, is not required.) The EB stepper utilizes a reticle on which the pattern is divided into a large number of exposure units, usually termed xe2x80x9csubfields,xe2x80x9d each defining a respective portion of the pattern. Each subfield typically is much larger (up to about 1-mm square on the reticle) than a single xe2x80x9ccell,xe2x80x9d requiring a correspondingly larger-diameter beam. Hence, since more of the pattern is transferred in each exposure xe2x80x9cshot,xe2x80x9d divided-reticle-reduction microlithography typically achieves a much higher throughput than cell-projection.
One type of reticle suitable for use in an EB stepper is a so-called xe2x80x9cscattering-stencilxe2x80x9d reticle. In a scattering-stencil reticle each subfield includes a respective portion of the reticle membrane, wherein the subfields are separated from one another by non-patterned regions occupied by support struts. The reticle membrane constitutes an electron-scattering body. In each subfield, the respective pattern elements are defined by a corresponding arrangement of apertures that freely transmit electrons of an incident beam. In other words, regions of the subfield in which electron transmission without scattering is desired are defined as respective apertures in the membrane, while the remaining regions of the membrane are respective regions in which electron transmission is accompanied by large forward-scattering. Most of the electrons that are forward-scattered during passage through the membrane are blocked by a downstream xe2x80x9cscatteringxe2x80x9d aperture. Consequently, the electrons reaching the surface of the substrate are more or less exclusively the electrons that have passed through the apertures only.
Another reticle type used in divided-reticle-reduction EB microlithography is the so-called xe2x80x9cscattering-membranexe2x80x9d reticle. A scattering-membrane reticle is divided into subfields in a manner similar to the scattering-stencil reticle. However, the membrane in the scattering-membrane reticle does not define pattern elements by corresponding apertures. Rather, the pattern elements are defined by corresponding regions of a highly scattering film formed on a relatively thin (thickness of 0.1 xcexcm or less) membrane through which incident electrons pass with substantially no scattering.
Generally, the percentage of incident electrons passing through the membrane of a scattering-membrane reticle is about 40%. This seemingly low number does not prevent attainment of sufficient contrast for good imaging of the pattern on the substrate. However, such reticles do pose a risk of chromatic aberration caused by forward-scattering of electrons during passage through the membrane. Chromatic aberrations can affect pattern resolution adversely.
A key operational goal of EB steppers is an ability to perform mass-production of semiconductor wafers, especially high-density memory chips (e.g., DRAMs having a memory capacity of at least 16 Gbits). To achieve such performance, the EB stepper must exhibit a correspondingly high level of pattern resolution. Examples of factors that contribute to such resolution achieved by an EB stepper include: (1) high acceleration of the electron beam, (2) low geometrical aberrations, and (3) high suppression of Coulomb effects and other resolution-destroying effects by the EB optical system of the stepper. Reduction of Coulomb effects is very important. In an EB stepper, whereas it is desirable to increase the beam current as much as possible in order to maximize productivity, increasing the beam current correspondingly increases the density of electrons in the beam. In conditions of high electron density, the repulsive forces between adjacent electrons in the beam are stronger than in lower-density beams. The resulting mutual repulsion of electrons away from each other in the beam is termed the xe2x80x9cCoulomb effect.xe2x80x9d During microlithography performed under such conditions, electrons reaching the surface of the substrate produce an image exhibiting a characteristic blur that degrades the resolution of the projected image.
In view of the problems summarized above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography apparatus and methods that exhibit better control of Coulomb effects as manifest in a pattern as transferred from a segmented reticle to a sensitive substrate.
In one embodiment of the method, the segmented reticle can be either a scattering-stencil reticle or a scattering-membrane reticle. Selected regions of the reticle are individually illuminated with a CPB illumination beam to produce a corresponding patterned beam. Most of the charged particles in the patterned beam that are highly scattered during passage through the reticle are blocked by a contrast aperture from reaching the sensitive substrate. Meanwhile, charged particles that are not scattered and/or weakly scattered during passage through the reticle pass through the contrast aperture and are focused as a projected image on the sensitive substrate. For exposing the pattern, the beam current of the patterned beam reaching the sensitive substrate is reduced, relative to the beam current actually passing through the reticle, to 50% or less. This reduction is performed by: (a) establishing the pattern, as defined on the reticle, as a normal-tone pattern or as an inverted-tone pattern, and (b) establishing the resist on the substrate as a positive or negative resist.
If the reticle is a scattering-stencil reticle (in which the pattern elements are defined by respective non-scattering, CPB-transmissive apertures in a highly CPB-scattering reticle membrane), then, in step (a), above, the beam current reaching the sensitive substrate is reduced by establishing an opening ratio of 50% or less for the pattern as a whole as defined on the reticle. The opening ratio is expressed as: 100 [Ao/(Ao+Ahs)], wherein Ao is the total area of the reticle occupied by the non-scattering apertures in the reticle membrane, and Ahs is the total area of the reticle occupied by the highly scattering reticle membrane.
If the reticle is a scattering-membrane reticle (in which the pattern elements are defined by respective voids in a highly CPB-scattering layer on a weakly CPB-scattering yet CPB-transmissive reticle membrane), then, in step (a), above, the beam current reaching the sensitive substrate is reduced by establishing an opening ratio of 50% or less for the pattern as a whole as defined on the reticle. The opening ratio is expressed as 100 [Aws/(Aws+Ahs)], wherein Ahs is the total area of the reticle occupied by both the weakly scattering reticle membrane and the highly scattering layer, and Aws is the total area of the reticle occupied by only the weakly scattering reticle membrane.
If the pattern as defined on the reticle cannot be established as a normal-tone or inverted-tone pattern so as to reduce the beam current to 50% or less, then either of the following can be performed: (1) splitting the reticle into multiple complementary reticles each defining a respective portion of the pattern, or (2) defining the pattern using a scattering-membrane reticle.
In another embodiment of the method, the pattern is defined on a segmented reticle that is either a scattering-stencil reticle or a scattering-membrane reticle. Selected regions of the reticle are individually illuminated with a CPB illumination beam to produce a corresponding patterned beam. At least most of the charged particles in the patterned beam that are highly scattered during passage through the reticle are blocked from reaching the sensitive substrate. Charged particles that are not scattered and/or that are weakly scattered during passage through the reticle are focused as a projected image on the sensitive substrate. With respect to regions of the reticle defining critical features of the pattern, the beam current of the patterned beam reaching the sensitive substrate is reduced, relative to a beam current actually passing through the reticle, to 50% or less by establishing the pattern, as defined on the reticle, as a normal-tone pattern or as an inverted-tone pattern, and by establishing the resist on the substrate as a positive or negative resist.
If the reticle is a scattering-stencil reticle, then the beam current reaching the sensitive substrate can be reduced, with respect to regions of the reticle defining critical features of the pattern, by establishing an opening ratio of 50% or less for the pattern as a whole as defined on the reticle. The opening ratio is expressed as 100[Ao/(Ao+Ahs)], wherein Ao is a total area of the reticle occupied by the non-scattering apertures in the reticle membrane, and Ahs is a total area of the reticle occupied by the highly scattering reticle membrane.
If the reticle is a scattering-membrane reticle, then the beam current reaching the sensitive substrate can be reduced, with respect to regions of the reticle defining critical features of the pattern, by establishing an opening ratio of 50% or less for the pattern as a whole as defined on the reticle. The opening ratio is expressed as 100[Aws/(Aws+Ahs)], wherein Ahs is a total area of the reticle occupied by both the weakly scattering reticle membrane and the highly scattering layer, and Aws is a total area of the reticle occupied by only the weakly scattering reticle membrane.
If the pattern as defined on the reticle cannot be established as a normal-tone or inverted-tone pattern so as to reduce the beam current to 50% or less, then either the reticle can be split into multiple complementary reticles each defining a respective portion of the pattern, or the pattern can be defined using a scattering-membrane reticle.
In yet another embodiment of a method for performing CPB microlithography, the pattern is defined on a segmented reticle that is either a scattering-stencil reticle or a scattering-membrane reticle. The reticle is divided into multiple subfields each defining a respective portion of the pattern. The subfields are individually illuminated using a CPB illumination beam to produce a corresponding patterned beam directed to the sensitive substrate. At least most of the charged particles in the patterned beam that are highly scattered during passage through the reticle are blocked from reaching the sensitive substrate. Charged particles that are not scattered and/or weakly scattered during passage through the reticle are projected as a projected image on the sensitive substrate. The subfield images are stitched together on the substrate so as to imprint a complete pattern on the substrate. For exposing the pattern, the beam current of the patterned beam reaching the sensitive substrate is reduced, relative to a beam current actually passing through the reticle, to 50% or less. This reduction is achieved by establishing the pattern, as defined on the reticle, as a normal-tone pattern or as an inverted-tone pattern, and establishing the resist on the substrate as a positive or negative resist. According to an exposure rule, the pattern is defined in a normal tone on the reticle and a negative resist is used on the substrate whenever (xcex7max+xcex7min)/2xe2x89xa650%. In this expression, xcex7 is a mean pattern-element density for the entire pattern, xcex7max is a maximum pattern-element density for each subfield, and xcex7min is a minimum pattern-element density for each subfield. Alternatively, the pattern is defined in an inverted tone on the reticle and a positive resist is used on the substrate whenever (xcex7max+xcex7min)/2 greater than 50%.
If the pattern as defined on the reticle cannot be established as a normal-tone or inverted-tone pattern, then the reticle can be split into multiple complementary reticles each defining a respective portion of the pattern. In this instance, each of the complementary reticles is exposed according to the exposure rule.
The method can include the step of changing the pattern-element density on the reticle by disposing dummy elements and/or unresolvable elements on the reticle.
According to yet another embodiment of a microlithography method, the pattern is defined on a segmented reticle that is either a scattering-stencil reticle or a scattering-membrane reticle. The reticle is divided into multiple subfields each defining a respective portion of the pattern. The subfields are individually illuminated with a CPB illumination beam to produce a corresponding patterned beam directed to the sensitive substrate. At least most of the charged particles in the patterned beam that are highly scattered during passage through the reticle are blocked from reaching the sensitive substrate. Charged particles that are not scattered and/or weakly scattered during passage through the reticle are projected as a projected image on the sensitive substrate. The subfield images are stitched together on the substrate so as to imprint a complete pattern on the substrate. With respect to subfields defining critical features of the pattern, a beam current of the patterned beam reaching the sensitive substrate is reduced, relative to a beam current actually passing through the reticle, to 50% or less. This reduction is achieved by establishing the pattern, as defined on the reticle, as a normal-tone pattern or as an inverted-tone pattern, and establishing the resist on the substrate as a positive or negative resist. The pattern is defined in a normal tone on the reticle and a negative resist is used on the substrate whenever (xcex7cmax+xcex7cmin)/2xe2x89xa650%. In this expression xcex7c is a mean pattern-element density for all subfields of the pattern that include critical features, xcex7cmax is a maximum pattern-element density for each subfield that includes critical features, and xcex7cmin is a minimum pattern-element density for each subfield that includes critical features. The pattern is defined in an inverted tone on the reticle and a positive resist is used on the substrate whenever (xcex7cmax+xcex7cmin)/2 greater than 50%.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.